The invention is intended for use in large scale integrated (LSI) circuits using metal oxide semiconductor (MOS) technology and in particular silicon gate MOS. The invention generally relates to a read only memory (ROM) that can be laser programmed after construction to create an LPROM that can be associated with the other devices in an LSI circuit. In particular, complementary (CMOS) technology is employed to fabricate the LPROM in a form that permits programming during the LSI manufacturing process.
PROMs are well-known in the prior art and many diverse forms have been developed commercially. Some are field programmable by the user for a particular application and some are programmed by the manufacturer in accordance with a user requirement. Most commonly, user programming is achieved by blowing fuse links in the memory array at selected memory locations. This is most useful in bipolar transistor circuits in which high current densities are easily achieved. However, in MOS technologies, the fuse blowing technique is difficult to achieve and its use can produce undesirable side effects in associated circuitry. Another form of PROM employs a predetermined metal mask configuration that is designed to determine the memory content. Thus, the memory function is predetermined and created in the manufacturing process. This approach does not permit modifying the memory content to accommodate the characteristics of the associated circuitry.